The induction of immunological memory to serogroup A and C polysaccharides in UK infants immunized with three doses of a meningococcal A/C oligosaccharide CRM197 conjugate vaccine was investigated. Forty UK infants vaccinated previously with three doses of a meningococcal A/C oligosaccharide-CRM197 conjugate vaccine at 2, 3 and 4 months of age, were revaccinated at a mean age of 145·6 weeks with either a 10 or 50μg dose of licensed meningococcal A/C polysaccharide vaccine. Serogroup-specific antibody and serum bactericidal antibody (SBA) responses were measured by enzyme-linked immunosorbent assay and serum bactericidal assays, respectively. Following challenge, anti-serogroup A and C polysaccharide antibody levels rose from pre-booster geometric mean concentrations (GMC) of 3·1 and 2·1 μg/ml respectively to 19·6 and 21·0 μg/ml 1 month post-booster. Serum bactericidal antibody geometric mean titres (GMTs) for serogroups A and C increased 156- and 113-fold from 2·1 and 7·1 pre-booster respectively to 327·4 and 800·7 post-booster. A serogroup A control group of 45 children received a 10 μg dose of licensed meningococcal A/C polysaccharide vaccine (with no prior history of serogroup A vaccination) had serogroup A SBA GMTs of 2·3 pre-vaccination rising to 8 post-vaccination with corresponding GMCs of 0·8 and 10·8 μg/ml. These rises in SBA following serogroup A/C conjugate vaccination are indicative of immunological priming.